Low-Temperature Cross-Linking Benzocyclobutene Based Polymer Dielectric for Organic Thin Film Transistors on Plastic Substrates.

The synthesis of a new benzocyclobutene based polymer, PSBBB, designed as a dielectric material for use in organic thin film transistors was reported. Compared to conventional benzocyclobutene-based materials, the introduction of a butoxide substituent at the 7-position of the benzocyclobutene pendant unit on the polymer allowed PSBBB to be cross-linked at temperatures of 120 °C, thus rendering it compatible with the processing requirements of flexible plastic substrates. The cross-linking behavior of PSBBB was investigated by Fourier transform infrared spectroscopy and differential scanning calorimetry, demonstrating cross-linking of the polymer after curing at 120 °C. Bottom-gate bottom-contact organic thin film transistors were fabricated using PSBBB as dielectric, affording a performance comparable to that of other dielectric polymeric materials.

High-Hole-Mobility Fiber Organic Electrochemical Transistors for Next-Generation Adaptive Neuromorphic Bio-Hybrid Technologies.

The latest developments in fiber design and materials science are paving the way for fibers to evolve from parts in passive components to functional parts in active fabrics. Designing conformable, organic electrochemical transistor (OECT) structures using poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) fibers has excellent potential for low-cost wearable bioelectronics, bio-hybrid devices, and adaptive neuromorphic technologies. However, to achieve high-performance, stable devices from PEDOT:PSS fibers, approaches are required to form electrodes on fibers with small diameters and poor wettability, that leads to irregular coatings. Additionally, PEDOT:PSS-fiber fabrication needs to move away from small batch processing to roll-to-roll or continuous processing. Here, it is shown that synergistic effects from a superior electrode/organic interface, and exceptional fiber alignment from continuous processing, enable PEDOT:PSS fiber-OECTs with stable contacts, high µC* product (1570.5 F cm-1 V-1 s-1 ), and high hole mobility over 45 cm2 V-1 s-1 . Fiber-electrochemical neuromorphic organic devices (fiber-ENODes) are developed to demonstrate that the high mobility fibers are promising building blocks for future bio-hybrid technologies. The fiber-ENODes demonstrate synaptic weight update in response to dopamine, as well as a form factor closely matching the neuronal axon terminal.

New Chemical Dopant and Counterion Mechanism for Organic Electrochemical Transistors and Organic Mixed Ionic-Electronic Conductors.

Organic mixed ionic-electronic conductors (OMIECs) have varied performance requirements across a diverse application space. Chemically doping the OMIEC can be a simple, low-cost approach for adapting performance metrics. However, complex challenges, such as identifying new dopant materials and elucidating design rules, inhibit its realization. Here, these challenges are approached by introducing a new n-dopant, tetrabutylammonium hydroxide (TBA-OH), and identifying a new design consideration underpinning its success. TBA-OH behaves as both a chemical n-dopant and morphology additive in donor acceptor co-polymer naphthodithiophene diimide-based polymer, which serves as an electron transporting material in organic electrochemical transistors (OECTs). The combined effects enhance OECT transconductance, charge carrier mobility, and volumetric capacitance, representative of the key metrics underpinning all OMIEC applications. Additionally, when the TBA+ counterion adopts an "edge-on" location relative to the polymer backbone, Coulombic interaction between the counterion and polaron is reduced, and polaron delocalization increases. This is the first time such mechanisms are identified in doped-OECTs and doped-OMIECs. The work herein therefore takes the first steps toward developing the design guidelines needed to realize chemical doping as a generic strategy for tailoring performance metrics in OECTs and OMIECs.

Enhancing the Backbone Coplanarity of n-Type Copolymers for Higher Electron Mobility and Stability in Organic Electrochemical Transistors.

Electron-transporting (n-type) conjugated polymers have recently been applied in numerous electrochemical applications, where both ion and electron transport are required. Despite continuous efforts to improve their performance and stability, n-type conjugated polymers with mixed conduction still lag behind their hole-transporting (p-type) counterparts, limiting the functions of electrochemical devices. In this work, we investigate the effect of enhanced backbone coplanarity on the electrochemical activity and mixed ionic-electronic conduction properties of n-type polymers during operation in aqueous media. Through substitution of the widely employed electron-deficient naphthalene diimide (NDI) unit for the core-extended naphthodithiophene diimide (NDTI) units, the resulting polymer shows a more planar backbone with closer packing, leading to an increase in the electron mobility in organic electrochemical transistors (OECTs) by more than two orders of magnitude. The NDTI-based polymer shows a deep-lying lowest unoccupied molecular orbital level, enabling operation of the OECT closer to 0 V vs Ag/AgCl, where fewer parasitic reactions with molecular oxygen occur. Enhancing the backbone coplanarity also leads to a lower affinity toward water uptake during cycling, resulting in improved stability during continuous electrochemical charging and ON-OFF switching relative to the NDI derivative. Furthermore, the NDTI-based polymer also demonstrates near-perfect shelf-life stability over a month-long test, exhibiting a negligible decrease in both the maximum on-current and transconductance. Our results highlight the importance of polymer backbone design for developing stable, high-performing n-type materials with mixed ionic-electronic conduction in aqueous media.

Water stable molecular n-doping produces organic electrochemical transistors with high transconductance and record stability.

From established to emergent technologies, doping plays a crucial role in all semiconducting devices. Doping could, theoretically, be an excellent technique for improving repressively low transconductances in n-type organic electrochemical transistors - critical for advancing logic circuits for bioelectronic and neuromorphic technologies. However, the technical challenge is extreme: n-doped polymers are unstable in electrochemical transistor operating environments, air and water (electrolyte). Here, the first demonstration of doping in electron transporting organic electrochemical transistors is reported. The ammonium salt tetra-n-butylammonium fluoride is simply admixed with the conjugated polymer poly(N,N'-bis(7-glycol)-naphthalene-1,4,5,8-bis(dicarboximide)-co-2,2'-bithiophene-co-N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide), and found to act as a simultaneous molecular dopant and morphology-additive. The combined effects enhance the n-type transconductance with improved channel capacitance and mobility. Furthermore, operational and shelf-life stability measurements showcase the first example of water-stable n-doping in a polymer. Overall, the results set a precedent for doping/additives to impact organic electrochemical transistors as powerfully as they have in other semiconducting devices.

On the Role of Contact Resistance and Electrode Modification in Organic Electrochemical Transistors.

Contact resistance is renowned for its unfavorable impact on transistor performance. Despite its notoriety, the nature of contact resistance in organic electrochemical transistors (OECTs) remains unclear. Here, by investigating the role of contact resistance in n-type OECTs, the first demonstration of source/drain-electrode surface modification for achieving state-of-the-art n-type OECTs is reported. Specifically, thiol-based self-assembled monolayers (SAMs), 4-methylbenzenethiol (MBT) and pentafluorobenzenethiol (PFBT), are used to investigate contact resistance in n-type accumulation-mode OECTs made from the hydrophilic copolymer P-90, where the deliberate functionalization of the gold source/drain electrodes decreases and increases the energetic mismatch at the electrode/semiconductor interface, respectively. Although MBT treatment is found to increase the transconductance three-fold, contact resistance is not found to be the dominant factor governing OECT performance. Additional morphology and surface energy investigations show that increased performance comes from SAM-enhanced source/drain electrode surface energy, which improves wetting, semiconductor/metal interface quality, and semiconductor morphology at the electrode and channel. Overall, contact resistance in n-type OECTs is investigated, whilst identifying source/drain electrode treatment as a useful device engineering strategy for achieving state of the art n-type OECTs.

Addition of the Lewis Acid Zn(C6 F5 )2 Enables Organic Transistors with a Maximum Hole Mobility in Excess of 20 cm2 V-1 s-1.

Incorporating the molecular organic Lewis acid tris(pentafluorophenyl)borane [B(C6 F5 )3 ] into organic semiconductors has shown remarkable promise in recent years for controlling the operating characteristics and performance of various opto/electronic devices, including, light-emitting diodes, solar cells, and organic thin-film transistors (OTFTs). Despite the demonstrated potential, however, to date most of the work has been limited to B(C6 F5 )3 with the latter serving as the prototypical air-stable molecular Lewis acid system. Herein, the use of bis(pentafluorophenyl)zinc [Zn(C6 F5 )2 ] is reported as an alternative Lewis acid additive in high-hole-mobility OTFTs based on small-molecule:polymer blends comprising 2,7-dioctyl[1]benzothieno [3,2-b][1]benzothiophene and indacenodithiophene-benzothiadiazole. Systematic analysis of the materials and device characteristics supports the hypothesis that Zn(C6 F5 )2 acts simultaneously as a p-dopant and a microstructure modifier. It is proposed that it is the combination of these synergistic effects that leads to OTFTs with a maximum hole mobility value of 21.5 cm2 V-1 s-1 . The work not only highlights Zn(C6 F5 )2 as a promising new additive for next-generation optoelectronic devices, but also opens up new avenues in the search for high-mobility organic semiconductors.

Enabling thin-film transistor technologies and the device metrics that matter.

The field-effect transistor kickstarted the digital revolution that propelled our society into the information age. One member of the now large family of field-effect devices is the thin-film transistor (TFT), best known for its enabling role in modern flat-panel displays. TFTs can be used in all sorts of innovative applications because of the broad variety of materials they can be made from, which give them diverse electrical and mechanical characteristics. To successfully utilize TFT technologies in a variety of rapidly emerging applications, such as flexible, stretchable and transparent large-area microelectronics, there are a number of metrics that matter.

Recent Progress in High-Mobility Organic Transistors: A Reality Check.

Over the past three decades, significant research efforts have focused on improving the charge carrier mobility of organic thin-film transistors (OTFTs). In recent years, a commonly observed nonlinearity in OTFT current-voltage characteristics, known as the "kink" or "double slope," has led to widespread mobility overestimations, contaminating the relevant literature. Here, published data from the past 30 years is reviewed to uncover the extent of the field-effect mobility hype and identify the progress that has actually been achieved in the field of OTFTs. Present carrier-mobility-related challenges are identified, finding that reliable hole and electron mobility values of 20 and 10 cm2 V-1 s-1 , respectively, have yet to be achieved. Based on the analysis, the literature is then reviewed to summarize the concepts behind the success of high-performance p-type polymers, along with the latest understanding of the design criteria that will enable further mobility enhancement in n-type polymers and small molecules, and the reasons why high carrier mobility values have been consistently produced from small molecule/polymer blend semiconductors. Overall, this review brings together important information that aids reliable OTFT data analysis, while providing guidelines for the development of next-generation organic semiconductors.

Remarkable Enhancement of the Hole Mobility in Several Organic Small-Molecules, Polymers, and Small-Molecule:Polymer Blend Transistors by Simple Admixing of the Lewis Acid p-Dopant B(C6F5)3.

Improving the charge carrier mobility of solution-processable organic semiconductors is critical for the development of advanced organic thin-film transistors and their application in the emerging sector of printed electronics. Here, a simple method is reported for enhancing the hole mobility in a wide range of organic semiconductors, including small-molecules, polymers, and small-molecule:polymer blends, with the latter systems exhibiting the highest mobility. The method is simple and relies on admixing of the molecular Lewis acid B(C6F5)3 in the semiconductor formulation prior to solution deposition. Two prototypical semiconductors where B(C6F5)3 is shown to have a remarkable impact are the blends of 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene:poly(triarylamine) (diF-TESADT:PTAA) and 2,7-dioctyl[1]-benzothieno[3,2-b][1]benzothiophene:poly(indacenodithiophene-co-benzothiadiazole) (C8-BTBT:C16-IDTBT), for which hole mobilities of 8 and 11 cm2 V-1 s-1, respectively, are obtained. Doping of the 6,13-bis(triisopropylsilylethynyl)pentacene:PTAA blend with B(C6F5)3 is also shown to increase the maximum hole mobility to 3.7 cm2 V-1 s-1. Analysis of the single and multicomponent materials reveals that B(C6F5)3 plays a dual role, first acting as an efficient p-dopant, and secondly as a microstructure modifier. Semiconductors that undergo simultaneous p-doping and dopant-induced long-range crystallization are found to consistently outperform transistors based on the pristine materials. Our work underscores Lewis acid doping as a generic strategy towards high performance printed organic microelectronics.

Small Molecule/Polymer Blend Organic Transistors with Hole Mobility Exceeding 13 cm(2) V(-1) s(-1).

A ternary organic semiconducting blend composed of a small-molecule, a conjugated polymer, and a molecular p-dopant is developed and used in solution-processed organic transistors with hole mobility exceeding 13 cm(2) V(-1) s(-1) (see the Figure). It is shown that key to this development is the incorporation of the p-dopant and the formation of a vertically phase-separated film microstructure.